The first cause of death in Japan is cancer and is steadily increasing, and radiation cancer treatment has attracted attention as a therapy method in Japan where improvement of the quality of life (QOL) is required in recent years. In order to improve the QOL as needs, radiation cancer treatment is beginning to spread widely in Japan, along with high precision of radiation cancer treatment which is a seed.
Radiation used for the treatment includes X-Rays, electron beams, proton beams, heavy particle beams, and neutron beams, but development of proton beam and heavy particle beam therapy systems has been particularly remarkable in recent years. Since the proton beams and the heavy particle beams can apply a dose concentrating on a tumor volume using properties that the energy is intensively deposited immediately before stopping to make a peak (black peak) of the dose, high-precision treatment with a minimal invasion can be expected. In the X-ray treatment, IMRT, IGRT, and the like have also been developed, and efforts to concentrate the dose rate on the tumor volume are progressing. As the radiotherapy system becomes more upgrading, it is required to improve precision of treatment planning, precision of patient positioning, and the overall precision related to radiation treatment up to the treatment planning and the measurement of the dose rate for QA of the system.
For the measurement of the dose rate in the radiation treatment, ionization chambers with good stability and reproducibility are widely used. The ionization chamber has limitation in downsizing due to its detection principle, and accordingly dose distribution measurement is performed using a semiconductor detector, which is relatively easy to downsize, instead of the ionization chamber. However, the semiconductor detector also has limitation in downsizing if a signal processing system is included. In addition, such a detector needs to apply a high voltage for measurement, and thus it is difficult to measure the dose rate by inserting into the body.
In addition, since such a detector has generally high-density and has large interaction with the radiation as compared with substances in the body or water, the influence of the detector itself cannot be ignored.
Therefore, as a detector capable of monitoring the dose rate in the body, a system having a scintillator and an optical fiber is effective, and the following techniques are known.
A radiation monitor disclosed in PTL 1 is characterized in that “using a scintillation fiber 2 emitting fluorescent light as a radiation detector, a combined one by fusion bonding to the scintillation fiber 2 and a light transmission fiber 3 for transmitting the fluorescent light which is made of a material same kind as this scintillation fiber 2 is connected to a light detector 4 changing the transmitted light to electrical pulse signal. The light emitted from the scintillation fiber 2 is detected by the light detector 4 of which signal is amplified with a preamplifier 5, measured with a measuring device 6 counting the electric pulse signal and this measured result is indicated with a display 7.”, and is to exactly measure the absorbed dose near a cancer lesion by inserting a dose meter into the body.
In PTL 1, the scintillation fiber is used as a scintillator, but an elastic body is preferable in consideration of invasiveness at the time of inserting into the body. Further, the scintillator is preferably processed so as to correspond to various shapes in the body.
A radiation monitor disclosed in PTL 2 is characterized in that “the radiation monitor radiates fluorescence by accepting radiation, and the scintillation fiber 11 having flexibility is used for the detector part of the radiation. When the scintillation fiber 11 emits the fluorescence by receiving the radiation, the emitted fluorescent light is transmitted by the optical fiber 4 of the after stage to the photoelectric convertor 2 of the after stage, the light is converted into the electric signal by the photoelectric convertor 2, then inputted into the operation device 3 of after stage. In the operation device 3, the dosage rate of the radiation incident on the detection part is calculated from the electric signal, and displayed with its value of the dosage rate on the display device 10.”, and is to measure the dose rate in a minute place using the scintillation fiber having flexibility.
In PTL 2, since the scintillation fiber having flexibility is used as the scintillator, the invasiveness at the time of inserting into the body is reduced. However, the scintillation fiber of PTL 2 does not have a wide dynamic range from low dose to high dose, so it has a problem that measurement accuracy is low.